This invention relates to cement dispersion agents comprising water soluble vinyl copolymer.
Hydraulic cement compositions such as concrete are a mixture of cement, gravel, sand, etc. kneaded together with water and harden by hydration of cement and a dispersant is frequently used as an adjuvant for this process. A dispersing agent is for improving workability, accelerating the hydration reaction of cement and for causing water reducing effects by improving dispersion and fluidity characteristics of cement particles in water such that the strength of concrete, etc. can eventually be improved. For this reason, a cement dispersion agent must satisfy the following requirements: (1) Dispersion and fluidity of cement particles must be large, (2) Change in the dispersion and fluidity characteristics over time must be small (or the slump loss must be small), (3) There is no excessive entrained air which may lead to the lowering of the strength of hardened concrete, and (4) It must be economical.
In the past, many proposals such as lignin sulfonates, hydroxy carboxylates, condensation products of naphthalene sulfonic acid and formaldehyde, condensation products of aromatic sulfonic acid and formaldehyde, condensation products of melamine sulfonic acid and formaldehyde, etc. have been made as cement dispersion agent. It is known, however, that each of these has not only its own advantages but also its own significant problems.
On the other hand, there have been proposals for making use of water-soluble vinyl copolymers as cement dispersion agents (Japanese Patent Publication Tokko No. 59-18338, Japanese Patent Publications Tokkai No. 62-70252, 62-78137 and 62-212252 and U.S. Pat. Nos. 4,036,660, 4,076,699 and 4,473,406). Although some of the already proposed water-soluble vinyl copolymers have superior characteristics related to dispersibility and fluidity initially, changes in these characteristics with time are large with these prior art water-soluble vinyl copolymers. Moreover, their production process is too complicated to be practical.
Water-soluble vinyl copolymers are produced by copolymerization of various vinyl monomers in water. In order that the water-soluble vinyl copolymer thus produced can be fully usable as a cement dispersion agent, however, polar groups capable of adsorbing to cement particles to provide dispersion and fluidity effects such as carboxylic acid group, sulfonic acid group and polyoxyalkylene group must be present within the copolymer chain at appropriate density ratios and it is also required that the copolymer have an appropriate molecular weight. If a water-soluble vinyl copolymer is produced by copolymerization in a water solution, however, those with molecular weights too large to be desirable for a cement dispersion agent tend to be produced. It is known for the purpose of adjusting the molecular weight of a water-soluble vinyl copolymer to use as a part of the solvent a chain transfer agent such as a water-soluble thiol compound such as thioglycolates and thioglycerine or a lower alcohol such as methanol and isopropanol. It is also known, for adjusting the molecular weight without using a chain transfer agent, to raise the polymerization temperature nearly to the boiling point of the solvent. If a water-soluble thiol compound is used as a chain transfer agent, however, a bad odor often stays with the produced water-soluble vinyl copolymer and it also tends to decay in storage. In order to prevent them, a complicated refinement process is required. If use is made of a lower alcohol as a part of the solvent, on the other hand, a large amount of the lower alcohol tends to remain within the produced water-soluble vinyl copolymer, adversely affecting the efficacy as cement dispersion agent. If the polymerization temperature is raised nearly to the level of the boiling point of the solvent, furthermore, it becomes extremely difficult to control the heat of polymerization and the speed of polymerization reaction, thereby adversely affecting workability and safety of operation and causing defects and irregularities in the quality of produced water-soluble vinyl copolymer.